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Fig. 3. A schematic of the device used for measuring frictional forces between the
file (dark red) and plectrum (pink). The plectrum was mounted on a linear
stage, which moved the plectrum over the file at velocity (V)
(directed into/out of the page). A force beam was mounted on the vertical beam
to measure the frictional force, Ff, which is directed
opposite to the movement of the plectrum. A second force beam was mounted on a
manual translation stage with which the normal force (Fn),
applied orthogonally between the two surfaces, could be varied. The inset
shows the directions of the force vectors; note that this drawing is rotated
90° relative to the apparatus diagram.